Generally, hexavalent chromium-containing residues are sticky and have high moisture content and it is difficult to dissolve and separate the hexavalent chromium from the residues by direct leaching. However, chromium-containing residues are subjected to weathering, washing and soaking by rainwater for long period during the transport arid outdoor pilling, and a large amount of hexavalent chromium ions will be wasted gradually and released to the environment, thereby polluting the underground water, rivers and lakes, and seriously affecting aquatic products, farmlands and various creatures, and, then humankind. On the other hand, chromium is a strategic metal which ranks first. Chromium and its compounds may be extensively used in various fields of the industrial manufacture, belonging to a necessary raw material in metallurgical industry, metal processing, electroplating, leather making, textile, dye and fire retardant materials. However, China is lack of chromium mine resources, with a present proven reserves accounting for 0.825% of a total reserves worldwide, belonging to a scarce resource. Therefore, treatment of hexavalent chromium-containing residues becomes to challenge requiring to be solved urgently in environmental protection. A relatively ideal method can not only solve the problem of chromium pollution, but also turn waste into treasure, harm into good and achieve resources as the pollution is reduced, promoting development and environmental protection simultaneously.
Aiming at the treatment of chromium-containing residues, overseas and domestic researches have developed various treatment methods, of which a main concept is to convert the hexavalent chromium which is soluble and highly toxic into trivalent chromium and to bury it. Wet detoxification is mostly used. However, such method has drawbacks that the chromium cannot be recycled, the reduced trivalent chromium will be oxidized to hexavalent chromium as the surroundings change, with incomplete detoxification, and it is difficult to apply the treated residues and takes lots of lands for stockpiling. Industrially, chromium-containing residues are often used as raw materials for producing architectural ornament materials, ceramics and glass-ceramics. Chromium-containing residues can be effectively used, but the chromium resource cannot be effectively recycled.
A preferred method is to leach and separate the hexavalent chromium and then treat the filtered residues, so that not only chromium can be recycled but also the filtered residues can be used comprehensively. CN patents “Process flow for treating leached-out chromium residue” (CN1085832A) and “Method for comprehensive uses by dissolving chromium residue to eliminate toxicity” (CN1201836A), leach out water-soluble hexavalent chromium directly with water, CN patent “method for recycling hexavalent chromium resource from chromium-containing residues” (CN102191390A) leaches out water-soluble and acid-soluble hexavalent chromium with sulfuric acid or hydrochloric acid at normal temperature, improving the leaching-out of hexavalent chromium accordingly, but with 60%-80% of hexavalent chromium resource being collected from the chromium-containing residues, which cannot recycle the chromium resources efficiently or detoxify the chromium-containing residues completely, with an environmental hidden danger remained, CN patent “Process for processing, recycling and reusing chromium-containing residues and effluent” (CN102699006A) nearly completely dissolves the chromium-containing residues with acid at normal temperature, then adds different precipitants respectively, and precipitates and separates mixed ions in the solution. Such method demands a big consumption of chemical reagents such as acid and precipitant, with complicated process and relatively high treatment cost.